Columnar-grained polycrystalline solar cell and process of manufacture

ABSTRACT

The invention relates to techniques for manufacturing columnar-grained polycrystalline sheets which have particular utility as substrates or wafers for solar cells. The sheet is made by applying granular silicon to a setter material which supports the granular material. The setter material and granular silicon are subjected to a thermal profile all of which promote columnar growth by melting the silicon from the top downwardly. The thermal profile sequentially creates a melt region at the top of the granular silicon and then a growth region where both liquid and a growing polycrystalline sheet layer coexist. An annealing region is created where the temperature of the grown polycrystalline silicon sheet layer is controllably reduced to effect stress relief.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 07/959,009, filedOct. 9, 1992, now U.S. Pat. No. 5,336,335.

BACKGROUND OF THE INVENTION

Photovoltaic solar cells are semiconductor devices which convertsunlight into electricity. Solar cells based on crystalline siliconoffer the advantage of high performance and stability. The principalbarrier to expanded utilization of silicon solar cells for electricpower generation is the present high cost of the solar cells.

In conventional solar cells based on single crystal or large grainpolycrystalline silicon ingot processes, the major cost factor isdetermined by the requirement of sawing ingots into wafers. Sawing is anexpensive processing step, and furthermore results in the loss ofapproximately half the costly ingot material as silicon dust. Theproblem to be solved requires the development of a low-cost process,that efficiently employs low-cost materials while maintaining solar cellperformance.

The technical requirements for a solution to the problem are based onthe achievement of a process that is controllable, has high arealthroughput, and generates material with adequate crystalline morphology.The prior art includes several processes which either effectivelyachieve controlled growth, or high areal throughput of silicon sheet orribbons. All these approaches eliminate the costly process of sawinglarge areas to create wafers from ingots. For example, publications byHopkins (WEB), Ettouney, et al. (EFG), Gurtler (RTR) and Eyer, et al.(SSP) describe processes that achieve controlled polycrystalline growthof grains greater than 1 mm in size at low linear speeds (andconsequently low areal generation rates). Common to these sheet growthprocesses is the fact that the sheet pulling direction and the directionof sheet growth are collinear. All of these processes employ a largetemperature gradient (>500 degrees Centigrade per centimeter) along thesheet growth direction. This gradient is necessary to achieve thepractical linear speed indicated (typically less than 2 cm/min), butalso introduces large thermal-induced stresses. In many cases thesestresses limit the practical sheet width that can be achieved by causingsheet deformations which make solar cell fabrication untenable. Thermalstresses can also create crystalline defects which limit solar cellperformance. Each of these processes attempts to achieve grain sizesthat are as large as possible in order to avoid the deleterious effectsof grain boundaries on solar cell performance.

Another set of processes has been developed that can achieve high arealthroughput rates. For example, publications by Bates, et al. (LASS),Helmreich, et al. (RAFT), Falckenberg, et al. (S-Web), Hide, et al.(CRP) and Lange, et al. (RGS) describe processes that achievepolycrystalline sheet growth with grain sizes in the 10 microns to 3 mmrange at high linear rates (10 to 1800 cm/min). Typically, theseprocesses have difficulty maintaining geometric control (width andthickness) (e.g. (LASS, RAFT, RGS), and/or experience difficulty withcontamination of the silicon by the contacting materials (e.g. RAFT,S-Web, CRP). Common to these sheet growth processes is the fact that thesheet pulling direction and the direction of crystalline growth in thesheet are nearly perpendicular. It is this critical feature of theseprocesses that allows the simultaneous achievement of high linearpulling speeds and reduced crystal growth speeds. Reduced crystal growthspeeds are necessary for the achievement of materials with highcrystalline quality.

The prior art regarding the fabrication of solar cells frompolycrystalline materials requires that the grain size be greater than1.0 mm. This requirement on grain size was necessitated by the need tominimize the deleterious effects of grain boundaries evident in priorart materials. Historically, small-grained polycrystalline silicon(grain size less than 1.0 mm) has not been a candidate for photovoltaicmaterial due to grain boundary effects. Grain boundary recombination ledto degradation of voltage, current and fill factors in the solar cell.Previous models, for example Ghosh (1980) and Fossum (1980), based onrecombination at active grain boundaries correctly predicted performanceof historical materials. By inference these models teach that theachievement of inactive grain boundaries permits the utilization ofsmall grained materials.

SUMMARY OF THE INVENTION

It is the object of this invention to provide a low cost process forforming low stream columnar-grained sheets that are employed in highperformance solar cells.

A further object of this invention is to provide techniques formanufacturing columnar-grained polycrystalline silicon sheets for use asa substrate in solar cells, which overcomes the disadvantages of theprior art.

A yet further object of this invention is to provide a process formanufacturing a low-cost solar cell that employs small-grainedpolycrystalline silicon with low-activity grain boundaries.

A still further object of this invention is to provide a substrate and asolar cell made from such process.

In accordance with this invention the sheet is formed by using acolumnar growth technique that controls the details of heat flow, andthus growth speed of the polycrystalline material. The process beginswith granular silicon that is applied to a setter material; the setterand silicon are then subjected to a designed thermal sequence whichresults in the formation of a columnar-grained polycrystalline siliconsheet at high areal throughput rates. The equipment employed toaccomplish the process includes a line source of energy and apolycrystalline sheet growth and annealing technology.

The invention may also be practiced with a process which includes a moredistributed source of energy application than a line source, such as bygraphite-based infrared heating.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE illustrates a perspective view showing the sequencefor fabricating low stress, columnar-grained silicon sheets usable assolar cells substrates in accordance with this invention.

DETAILED DESCRIPTION

The present invention is directed to techniques used for making improvedcolumnar grain polycrystalline sheets which are particularly adaptablefor use as substrates or wafers in solar cells. The ability to use thesheet as a solar cell substrate makes possible the provision of a solarcell consisting entirely of silicon material where the sheet wouldfunction as a substrate made of silicon and the remaining layers of thesolar cell would also be made of silicon.

The desired properties of the columnar-grained silicon sheet orsubstrate fabricated for inclusion in a low-cost solar cell inaccordance with the teachings of this invention are: flatness, a smoothsurface, minority carrier diffusion length greater than 40 microns,minimum grain dimension at least two times the minority carrierdiffusion length, low residual stress and relatively inactive grainboundaries. Since the minimum grain dimension of the columnar grainsilicon sheet is at least two times the minority carrier diffusionlength which in turn is greater than 40 microns, the columnar grainswould have a grain size greater than 80 microns. Grain sizes down to 10microns can be employed with minority carrier diffusion length greaterthan 10 microns, and will lead to solar cells having lower currents, andlower power. The desired properties of a process for fabricatingcolumnar-grain silicon material appropriate for inclusion in a low-costsolar cell in accordance with the teachings of this invention are: lowthermal stress procedure, controlled nucleation, high areal throughput,and simple process control.

The criteria for the columnar-grain silicon material product of flatnessand smoothness are required to make solar cell fabrication tenable. Therequirements on diffusion length and grain size are to minimizerecombination losses in the bulk and at grain surfaces (i.e. grainboundaries), respectively. The requirement of relatively inactive grainboundaries is to effect the minimization of grain boundaryrecombination. The requirement of low residual stress is to minimizemechanical breakage and to maintain high minority carrier diffusionlengths.

The criteria for the columnar-grained silicon process of a low thermalstress procedure is to effect minimization of bulk crystalline defects.The requirement of controlled nucleation is to affect the achievement ofthe required grain morphology and size. The criteria for high arealthroughput and simple process control arc to achieve low-cost andmanufacturability.

The single Figure is a perspective view illustrating the sequence forfabricating low stress, columnar-grained silicon sheets. The process asdepicted moves from left to right. In general, a setter material 100,which serves as a mechanical support, is coated with a granular siliconlayer 200, and is passed through a prescribed thermal profile. Theprescribed thermal profile first creates a melt region 300 at the top ofthe granular silicon 200, and then creates a growth region 400 whereboth liquid and a growing layer of polycrystalline layer coexist.Finally, there is an annealing region 500 where the temperature of thepolycrystalline silicon sheet layer 600 is reduced in a prescribedmanner to effect stress relief.

The setter material 100 is selected based on the following requirements.It must: maintain its shape during the sheet formation thermalprocessing; not chemically interact with, or adhere to, the siliconmaterial; and possess the proper thermal characteristics to effect therequired sheet growth and annealing. The form of the setter material mayeither be as a rigid board or as a flexible thin belt.

Several materials including, but not limited to, quartz, refractoryboards (e.g. silica and/or alumina), graphite, and silicon carbide havebeen employed and maintained the proper geometric shape during thermalprocessing.

To assure that the setter 100 does not adhere to the finalpolycrystalline silicon sheet 600, a release agent coating 110 isapplied to the setter. Either, or a combination of, silicon nitride,silicon oxynitride, silica, or alumina have been employed as this agent.A low-cost method for applying this coating is to form a liquid slurrythat is painted or sprayed on the bare setter, and then subsequentlydried in an oxidizing atmosphere before use. The release agentfacilitates separation of the sheet and permits reuse of the settermaterial.

In the process design the thermal characteristics of the setter 100 playa key role in managing the melt and growth processes. In the melt region300 it is preferred that the thermal conductivity of the setter be lowto assure the efficient deployment of the energy being used to melt thegranular silicon 200. The thermal properties of the setter may betailored to possess a strip of higher thermal conductivity under theouter edges 210 of the strip of granular silicon. The effect of thisstrip is to define the outer edges of the growing sheet. The thermalconductivity of the setter may also be tailored to assist in definingnucleation sites to commence growth. This can be accomplished by locallyplacing thermal shunts 120 in the setter. These shunts 120 provide athermal conduction path between the top and bottom of the setter,effecting a local path for removing the heat of solidification, andresult in sites where nucleated growth occurs.

In a preferred embodiment the setter material is low density 1.5 cmthick silica board. The setter preparation is completed by coating thetop surface with a release agent 110. This is accomplished using anaqueous colloidal solution of silicon nitride that is painted on the topsurface and baked in an oxidizing atmosphere to form a non-wetting,non-adhering oxynitride layer, before the initial application ofgranular silicon.

The granular silicon 200 is selected based on the followingrequirements. It must: be properly sized; be of adequate purity; andcontain a chemical ingredient to provide a p-type resistivity of thegrown silicon sheet 600 in the range of 0.1. .,.!. to 10 ohm-cm.

The range of proper sizes for the granular silicon 200 employed in theprocess is between 100 and 1000 micrometers. The upper limit isdetermined by the design thickness for the silicon sheet material. As arule the minimum dimension of the largest silicon particles should beequal to or less than the desired thickness of the silicon sheetmaterial. The lower size limit of the particle distribution is dependenton the dynamics of the melting process, and the need to limit the amountof silicon oxide. The silicon oxide is a source of sheet contamination,and naturally occurs at all silicon surfaces.

The purity level necessary in the sheet silicon is determined by therequirements for the efficient operation of a solar cell devicefabricated on the sheet. Whereas the employment of low-processedmetallurgical grade silicon is not adequate, utilization of highlyprocessed semiconductor grade silicon is not necessary. In practice, thepreferred process can be executed with off-grade semiconductor gradesilicon. It is also an advantage of the preferred process that anadditional degree of impurity reduction is accomplished during sheetgrowth by segregation of impurities to the sheet surface, where they mayeasily be removed by a subsequent chemical etch. This mechanism forpurification by segregation is operative in the preferred process as theactual crystalline growth rate is less than 0.1 cm/min in the crystalgrowth direction, comparable to that employed in the single crystalfloat zone process. This mechanism is not operative in sheet growthtechnologies that have the crystalline growth rate equal to the sheetpulling speed (approx., 2 cm/min). At these higher growth velocities,there is not sufficient time for effective segregation to occur betweenliquid and solid as the process is diffusion limited.

It is necessary to provide for the addition of a separate constituentin, or with, the granular silicon to effect in electrical resistivity inthe range of 0.1 to 10 ohm-cm in the sheet material. Typically, forp-type conductivity in the sheet material the preferred elements areboron, aluminum, or indium. As an example of the preferred embodiment,the addition of powdered boron silicide followed by mechanical mixing ofthe granular silicon provides for the accomplishment of the requiredp-type resistivity in the subsequently grown silicon sheet.

The properly doped p-type granular silicon 200 is uniformly layered onthe coated setter 100. For example, this process can be effectivelyaccomplished by using a doctor blade. The spacing between the edge ofthe doctor blade and the setter surface needs to be at least two timesthe minimum dimension of the largest particle in the granular siliconsize distribution. Furthermore, the thickness of the final silicon sheet600 can be as little as the minimum dimension of the largest particle inthe granular size distribution.

The silicon-coated setter is transported into an environmental chamberwith an argon or nitrogen overpressure. In a preferred embodiment amixture of argon and hydrogen gas is employed to effectively limit theamount of silicon oxide that is formed during the growth process. Thepercent of hydrogen employed is determined by the water vapor content inthe chamber. The ratio of hydrogen to water vapor controls the magnitudeof silicon oxide formation. The chamber may include a pre-heat zoneemployed to raise the temperature to 1100 to 1400° C., which incombination with the hydrogen present has the effect of reducing thenative oxide of silicon that exists on the granular silicon. Anoverpressure consisting of at least 10% nitrogen (by volume) is employedin any or all of the pre-heat, melting, growth and anneal zones.

After the granular silicon 200 has been pre-heated it is then broughtinto a thermal zone 300 where the top portion of the granular siliconlayer 200 is melted. In the preferred embodiment, this thermal zone andthe melting of the top portion of the layer is accomplished using afocussed beam of light. The length of the focussed beam along thedirection of setter motion is about 1 centimeter. The depth of thegranular silicon that is melted depends on the intensity of the inputenergy from thermal zone 300, the thickness of the granular siliconlayer, the linear speed of the granular silicon coated setter throughthermal zone 300, and the details of heat transfer between the granularsilicon 200 and the seer 100. The outer edges of the melt zone arestabilized by the thermal shunts 120 engineered into the setter 100 orby reducing energy intensity at the edges. These thermal shunts 120inhibit the depth of melting at the outer edges 210 and thus promoteedge stabilization. Between 25 and 90% (and preferably between 50% and90%) of the granular silicon depth is melted. The invention may also bepracticed where the entire layer may have experienced a liquid stateprior to solidification into a sheet. The material at the bottom of thegranular layer is partially melted by liquid silicon penetrating fromthe molten silicon layer above. This partially melted layer of siliconforms a net 220. Other materials incorporated in a substrate designed tobe thermally-matched to silicon can be employed as a non-reusable net220 material. Other materials including fabrics that are woven ornon-woven, such as graphite, can be employed as the net 220. Othergranular materials that are partially melted or unmelted, such assilicon carbide, can be employed as the net 220 material. The net 220 isresponsible for four key process features. First because it is wetted bythe molten silicon above, this layer stabilizes the melt and growthzones by defeating the surface tension of the molten silicon over-layer.This allows the production of wide sheets, with smooth surfaces. Second,this layer serves as a plane to nucleate subsequent growth. Third, thislayer minimizes molten silicon contact with the supporting setter andrelease coating, thereby minimizing any potential contamination byimpurities. Fourth, this layer serves as highly defected back plane,intrinsically gettering impurities from the active silicon layer above,allowing the employment of lower purity, lower-cost grades of siliconraw material.

Where the net 220 is made from a material such as graphite, the graphitecould be unrolled and applied over the setter material before thegranular silicon is applied. Thus, the net is between the granularsilicon and the setter material. The later melted silicon would functionas a nucleation site. The net would function to stabilize the melt,minimize molten silicon contact with the underlying setter and act as arelease coating. Any or all of the preheat, melting, growth, and annealthermal profiles for the granular powder and resultant sheet could beachieved by graphite based heater technology.

Where the net 220 is made from the materials incorporated in anon-reusable substrate designed to be thermal coefficient-matched tosilicon, the substrate could be positioned on the setter material beforethe silicon is applied. Thus, the non-reusable substrate is between thegranular silicon and the setter material. The non-reusable substratewould act as a nucleation site and stabilize the melt. Any or all of thepreheat, melting, growth and anneal thermal profiles for the granularpowder and resultant sheet could be achieved by graphite-based heatertechnology.

After leaving the melt creation zone 300 of the thermal profile, themelt pool on the partially melted silicon net 220 moves into the growthzone 400 of the thermal profile. In this zone the growth is initiated onthe silicon net 220. Because growth is nucleated from the partiallymelted silicon net 220, the grain size of the granular silicon 200 is animportant parameter in determining the size of the columnar grains inthe grown sheet 600. In the preferred embodiment, multi-grained orsingle crystal granular silicon 200 is used to achieve relatively largecolumnar grains (average grain size 0.002 to 1.0 cm) in the grown sheet600. In one embodiment, growth may also be preferentially initiated atsites 210 in the granular silicon layer where the beat transfer iscontrolled by thermal shunting areas designed in the setter. Thedirection of the growth front is approximately perpendicular to theplane of the setter. The length of the growth zone along the directionof setter motion is from 2 to 20 centimeters, and is slightly less thanthe entire length of the melt pool. The length of the growth zone isdetermined by controlling the rate of loss of heat (and therefore growthrate) attending the solidification process. As a consequence of thegrowth process, the grains that are grown are columnar in nature.Typically, individual grains in the resulting sheet 600 extend from thetop surface to the bottom, and are at least as wide as they are high.Sheet thicknesses in the range of 400 to 500 microns can be achieved atsleet pulling speeds in excess of 30 cm/min.

After leaving the growth zone 400 of the thermal profile, the sheet 600moves into the annealing zone 500 of the thermal profile. In this zonethe grown sheet, still at approximately 1400° C., is subjected to alinear temperature gradient along the direction of setter motion. Thelinear temperature profile eliminates buckling and cracking of theas-grown sheet, and minimizes the generation of dislocations. Thethickness of the grown sheet is in the range of 350 to 1000 microns inthe preferred process. Because the thickness of the final grown sheet600 is determined by the precise application of granular silicon 200 tothe setter 100, exceptional sheet thickness control and processstability are achieved in comparison to sheet technologies pulled from amelt, where thickness is controlled by the melt meniscus. After cooldown, the sheet is removed from the setter, and appropriately sized bysawing or scribing, for fabrication into solar cells. The setter isreused for making further columnar-grained polycrystalline sheets.

The properties of the sheet material fabricated with the above processare quite amenable to the fabrication of efficient solar cells. Thisprocess generates material that has unique properties of size andcharacter. Although the grains are columnar, and have average sizes inthe range of 0.002 to 1.000 cm in extent, solar cells fabricated onmaterial in the range of 0.01 to 0.10 cm may achieve voltages in excessof 560 mV, and fill factors in excess of 0.72. The achievement of thesevalues on such small . .ground.!. .Iadd.grained .Iaddend.materialindicate that this material is not being limited by recombination atgrain boundaries as had been previously predicted by Ghosh. Previously,columnar grains were dismissed as being ineffective since columnargrains were always small, and small grains were thought not to work. Theprocess herein described achieves columnar grams that yield materialwith relatively benign grain boundaries with the result that efficient,low-cost solar cells can be manufactured.

The process herein described can be carried out in a continuous manner,resulting in continuous sheets that can be appropriately sized using anin line scribe or a saw. Impurity content in the melt and grown sheetquickly reaches steady-state; it does not increase during continuousprocessing. Since all embodiments include application of granularsilicon to the setter, and since material enters the melt creation zonein this form, melt replenishment is not a problem, unlike sheettechnologies pulled from a melt pool. After being properly sized, thesheets function as a substrate by having the remaining layers formedthereon to produce solar cells. Where the remaining layers are ofsilicon, a completely silicon solar cell results.

What is claimed is:
 1. A process of making an improved columnar-grainedpolycrystalline sheet for functioning as a substrate for a solar cell,comprising (a) applying granular silicon to a setter material having arelease coating at its upper surface and which supports the granularsilicon, (b) preheating the setter material and granular silicon in apreheat zone, (c) subjecting the setter material and granular silicon toa thermal profile which causes melting of the granular silicon from thetop downwardly, wherein 25 to 90% of the granular silicon depth ismelted and the partially melted silicon below the melted materialfunctions as a net to stabilize the melt and to minimize molten siliconcontact with the underlying setter material and release coating and tonucleate subsequent crystal growth, (d) transporting the melt pool onthe silicon net into a growth zone wherein a thermal profile is createdto promote columnar growth of a columnar grain size greater than 20microns in a direction approximately perpendicular to the plane of thesetter material and where both liquid and a growing polycrystallinelayer coexist and where the entire layer experiences a liquid stateprior to solidification, (e) transporting the grown sheet into an annealzone wherein a linear temperature gradient along the direction of settermaterial motion is provided to promote low stress cooling of the sheet,(f) removing the polycrystalline sheet from the setter material,facilitated by a release agent, and (g) reusing the setter material forthe making of further columnar-grained polycrystalline sheet.
 2. Theprocess of claim 1 including achievement of any or all of the preheat,melting, growth, and anneal thermal profiles for the granular siliconand resultant sheet by focused light energy.
 3. The process of claim 1including creating an electrical resistivity in the sheet layer in therange of 0.1 to 10 ohm-cm by adding separate constituents to thegranular silicon.
 4. The process of claim 1 including the use ofgranular silicon sized between 100 to 1000 micrometers, which has apurity between metallurgical grade and electronic grade silicon.
 5. Theprocess of claim 1 including stabilizing the outer edges of the meltzone by thermal shunts, or reduced energy intensity at the edge of themelt.
 6. The process of claim 1 including forming nucleation sites inthe setter material to commence growth by locally placing thermal shuntsin the setter materialk to provide a thermal conduction path between thetop and the bottom of the setter material.
 7. The process of claim 1wherein multi-grained or single crystal granular silicon is used tonucleate columnar grains having an average gain size of 0.002 to 1 cm inthe subsequent sheet.
 8. The process of claim 1 wherein the settermaterial is selected from the group consisting of quartz, silica,alumina, graphite, and SiC.
 9. The process of claim 1 wherein the settermaterial is replaced by a thin belt material which supports the sheetduring formation and thermal processing, and does not chemicallyinteract with, or adhere to, the silicon material.
 10. The process ofclaim 1 wherein the resulting silicon sheet has the characteristics offlatness, a smooth surface, minority carrier diffusion length greaterthan 10 microns, low residual stress, and relatively inactive grainboundaries.
 11. The process of claim 1 including utilizing the sheet asa substrate for a solar cell by forming the additional solar cell layerson the substrate.
 12. A solar cell made by the process of claim
 11. 13.A substrate made by the process of claim
 1. 14. The process of claim 1wherein the partially melted silicon net and the setter material arereplaced by a non-melting, non-reusable, thermal coefficient-matchedsubstrate which is wetted by and stabilizes the molten siliconover-layer, nucleates subsequent growth, and serves as a supportingsubstrate during subsequent solar cell processing of the grown sheet.15. The process of claim 14 including utilizing the sheet as a substratefor a solar cell by forming the additional solar cell layers on thesubstrate.
 16. A solar cell made by the process of claim
 15. 17. Asubstrate made by the process of claim
 14. 18. The process of claim 14wherein graphite is used as the substrate.
 19. The process of claim 1including achievement of any or all of the preheat, melting, growth, andanneal thermal profiles for the granular silicon and resultant sheet bygraphite based heater technology.
 20. A process in claim 1 where anoverpressure consisting of at least 10% nitrogen (by volume) is employedin any or all of the pre-heat, melting, growth and anneal zones.
 21. Aprocess of making an improved columnar-grained polycrystalline sheet forfunctioning as a substrate for a solar cell, comprising (a) applying anet to a setter material, (b) applying granular silicon to the settermaterial and the net whereby to support the granular silicon, (c)preheating the setter material and the net and the granular silicon in apreheat zone, (d) subjecting the setter material and the net and thegranular silicon to a thermal profile which causes melting of thegranular silicon from the top downwardly, wherein 25 to 90% of thegranular silicon depth is melted and the partially melted silicon belowthe melted material functions as a nucleation site to nucleatesubsequent crystal growth and the net functions to stabilize the meltand to minimize molten silicon contact with underlying setter and thenet functions as a release coating, (e) transporting the melt pool onthe net into a growth zone wherein a thermal profile is created topromote columnar growth of a columnar grain size greater than 20 micronsin a direction approximately perpendicular to the plane of the settermaterial and where both liquid and a growing polycrystalline layercoexist and where the entire layer experiences a liquid state prior tosolidification, (f) transporting the grown sheet into an anneal zonewherein a linear temperature gradient along the direction of settermaterial motion is provided to promote low stress cooling of the sheet,(g) removing the polycrystalline sheet from the setter material,facilitated by a release agent, and (h) reusing the setter material forthe making of further columnar-grained polycrystalline sheets.
 22. Theprocess of claim 22 wherein the net is made from a graphite material.23. The process of claim 22 including achievement of any or all of thepreheat, melting, growth, and anneal thermal profiles for the granularsilicon and resultant sheet by graphite based heater technology.
 24. Theprocess of claim 21 wherein the net is made from silicon carbide. 25.The process of claim 21 including achievement of any or all of thepreheat, melting, growth, and anneal thermal profiles for the granularsilicon and resultant sheet by graphite based heater technology.
 26. Theprocess of claim 21 including utilizing the sheet as a substrate for asolar cell by forming the additional solar cell layers on the substrate.27. A solar cell made by the process of claim
 26. 28. A substrate madeby the process of claim
 21. 29. A process in claim 21 where anoverpressure consisting of at least 10% nitrogen (by volume) is employedin any or all of the pre-heat, melting, growth and anneal zones.
 30. Aprocess of making an improved columnar-grained polycrystalline sheet forfunctioning as a substrate for a solar cell, comprising (a) providing alayer of graphite material, (b) applying granular silicon to thegraphite material whereby to support the granular silicon, (c)preheating the graphite material and the granular silicon in a preheatzone, (d) subjecting the graphite material and the granular silicon to athermal profile which causes melting of the granular silicon from thetop downwardly, wherein 25 to 90% of the granular silicon depth ismelted and the partially melted silicon below the melted materialfunctions as a nucleation site to nucleate subsequent crystal growth andthe graphite material functions as a release coating, (e) transportingthe melt pool on the graphite material into a growth zone wherein athermal profile is created to promote columnar growth of a columnargrain size greater than 20 microns in a direction approximatelyperpendicular to the plane of the graphite material and where bothliquid and a growing polycrystalline layer coexist and where the entirelayer of experiences a liquid state prior to solidification, (f)transporting the grown sheet into an anneal zone wherein a lineartemperature gradient along the direction of graphite material motion isprovided to promote low stress cooling of the sheet, (g) removing of thepolycrystalline sheet from the graphite material, and (h) reusing thegraphite material for the making of further columnar-grainedpolycrystalline sheets.
 31. The process of claim 30 including utilizingthe sheet as a substrate for a solar cell by forming the additionalsolar cell layers on the substrate.
 32. A solar cell made by the processof claim
 31. 33. A substrate made by the process of claim
 30. 34. Aprocess in claim 30 where an overpressure consisting of at least 10%nitrogen (by volume) is employed in any or all of the pre-heat, melting,growth and anneal zones. .Iadd.
 35. A sheet of silicon, said siliconsheet having a pair of opposing manor surfaces, said silicon of saidsheet being of elongated columnar grain form with the grains having anaverage width in the range of 0.002 to 1 cm in size with the columnaraxis thereof extending in the direction of their columnar axis frommajor surface to major surface, and said sheet having a thickness offrom 350 to 1000 microns. .Iaddend..Iadd.36. The sheet of claim 35wherein said grain size is greater than 80 microns. .Iaddend..Iadd.37.The sheet of claim 36 wherein said sheet has an electrical resistivityin the range of 0.1 to 10 ohm-cm. .Iaddend..Iadd. The sheet of claim 37having the characteristics of flatness, a smooth surface, minoritycarrier diffusion length greater than 10 microns, low residual stress,and low activity grain boundaries. .Iaddend..Iadd.39. The sheet of claim38 wherein said minority carrier diffusion length is greater than 40microns. .Iaddend..Iadd.40. The sheet of claim 38 wherein the minimumgrain dimension is at least two times said minority carrier diffusionlength. .Iaddend..Iadd.41. The sheet of claim 40 wherein said sheet issized to function as a substrate. .Iaddend..Iadd.
 2. The sheet of claim35 wherein said sheet has an electrical resistivity in the range of 0.1to 10 ohm-cm. .Iaddend..Iadd.43. The sheet of claim 35 having thecharacteristics of flatness, a smooth surface, minority carrierdiffusion length greater than 10 microns, low residual stress, and lowactivity grain boundaries. .Iaddend..Iadd.44. The sheet of claim 43wherein the minimum grain dimension is at least two times said minoritycarrier diffusion length. .Iaddend..Iadd.45. The sheet of claim 35wherein said sheet is sized to function as a substrate..Iaddend..Iadd.46. The sheet of claim 35 wherein said grains from at oneof said surfaces having features of a silicon net. .Iaddend..Iadd.47. Asubstrate made from a sheet of silicon, said silicon sheet having a pairof opposing major surfaces, said silicon sheet being of elongatedcolumnar grain form with the grains having an average width in the rangeof 0.002 to 1 cm in size with the columnar axis thereof extending in thedirection of their columnar axis from one of said surfaces to the otherof said surfaces, and said silicon sheet having a thickness of from 350to 1000 microns. .Iaddend..Iadd.48. The substrate of claim 47 whereinsaid grain size is greater than 80 microns. .Iaddend..Iadd.49. In asolar cell having a substrate, the improvement being in that saidsubstrate comprises a sheet of silicon, said silicon sheet having a pairof opposing major surfaces, said silicon sheet being of elongatedcolumnar grain form with the grains having an average width in the rangeof 0.002 to 1 cm in size with the columnar axis thereof extending in thedirection of their columnar axis from one of said surfaces to the otherof said surfaces, and said silicon sheet having a thickness of from 350to 1000 microns. .Iaddend..Iadd.50. The solar cell of claim 49 whereinsaid substrate has a thickness in the range of 0.01 to 0.10 cm, and saidsolar cell being capable of achieving voltages in excess of 560 mV andfill factors in excess of 0.72. .Iaddend..Iadd.51. The solar cell ofclaim 50 wherein said solar cell includes a plurality of layers madeentirely of silicon supported by said substrate. .Iaddend.